This document summarizes key concepts in Mendelian genetics. It explains that Gregor Mendel studied trait inheritance in pea plants and discovered that traits can be dominant or recessive. Dominant traits always appear if present, while recessive traits only appear if an organism inherits two recessive alleles. An organism's genotype describes its genetic makeup, while its phenotype describes its physical traits. Punnett squares are used to predict the probability that offspring will inherit different combinations of alleles from their parents.

1. Biology
Unit 4 Notes:
Mendelian
Genetics

2. (1) Gregor Mendel
• Monk
• Studied genetics of pea plants.
• He noticed that:
– There were some traits which showed up all of
the time (dominant).
– There were some traits which showed up some
times (recessive).
– These traits are passed down from parents.
– You can’t tell just by looking at an organism,
which genes it has.

5. (2) Dominant + Recessive Traits
• Dominant Trait:
– These will ALWAYS SHOW UP if they are
present.
– Representation  Capital Letters
• Recessive Trait:
– These will ONLY SHOW UP if the organism
inherited 2 together.
– Representation  Lower Case Letters

7. (3) Inheriting Traits
• You inherit 2 alleles for every gene.
– Gene = Chunk of DNA.
– Allele = Variation that chunk can come in.
• “Heterozygous” = You inherited 1-Dominant and
1-Recessive allele.
• “Homozygous” = You inherited 2 of the same
alleles.
– “Homozygous Dominant” = You inherited 2-Dominant
alleles.
– “Homozygous Recessive” = You inherited 2-Recessive
alleles

9. (4) Ways to Describe Inheritance
• Phenotype:
– Physical Description.
– Examples:
• “White”
• “Brown”
• “White and Brown Spotted”
• Genotype:
– Genetic Description.
– Examples:
• AA or Homozygous Dominant
• Aa or Heterozygous
• aa or Homozygous Recessive

11. (5) What’s With These Letters?
• The letters represent the alleles an organism can
inherit.
• You should choose letters with a different Capital
+ Lower Case appearance:
– A and a
– B and b
– Bad Life Choices: S and s, X and x, C and c
• Actual Letters DO NOT MATTER:
– Heterozygous needs to be: Capital + Lower
– Homozygous Dominant needs to be: 2 Capital
– Homozygous Recessive needs to be: 2 Lower

13. (6) What Are Punnett Squares?
• Punnett Square = Tool used to make genetic
predictions.
• They do NOT tell you with certainty how many
offspring will have which traits!!!
• Tells you probability.
– “There is a ¼ or 25% chance that the offspring will….”
• SET-UP:
– 1 Parent’s genotype on the top
– Other Parent’s genotype on the side

14. (7) Setting Up Punnett Squares
• 1 Parent’s Genotype on the Top
(doesn’t matter which one).
• 1 Parent’s Genotype on the Side
(doesn’t matter which one).
• 1 Letter per Box!
• For Example:
– If one parent is AA and the other parent is aa…
Draw your Punnett Square!
A A
a
a

15. (8) Interpreting Punnett Squares
• Each Box = ¼ or 25% chance
• For Example:
– 4/4 Boxes have Aa, so…
– There is a 4/4 or 100% chance that the offspring will be
Heterozygous.
– I could also say that there is a 100% chance that the
offspring will look dominant.
Draw your Punnett Square!
A A
a
a
Aa Aa
Aa Aa